Purine nuceloside phosphorylase (PNP) catalyzes the phosphorolysis of purine nucleosides to yield free purines and ?-1-ribose phosphate. A genetic defect in humans that results in the lack of PNP, results in a complete loss of T-cells, but no other effects. Thus PNP is an excellent target for the therapy of a number of T-cell specific pathologies including T-cell lymphomas and graft rejection. We have recently collected data to better than 2.0E on a complex between calf spleen PNP and a 20 pM transition state-inhibitor. Refinement is underway and currently reveals several of the strategies used by this enzyme to stabilize the transition state, which includes a conformational rearrangement. This structure represents one of the highest affinity enzyme-ligand complexes solved to date and will form the basis for future inhibitor development and mechanistic studies.